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IMPERIAL COLLEGE LONDON Department of Earth Science and Engineering Centre for Petroleum Studies Shallow Seismic Analysis in Pagosa Springs, Colorado, USAby Junghee KimA report submitted in partial fulfilment of therequirements for the MScSeptember 2012DECLARATION OF OWN WORKI declare that this thesis is entirely my own work and that where any material could beconstrued as the work of others, this has been fully cited and referenced, and/or withappropriate acknowledgement given. Signature Name of student Junghee Kim Name of supervisor Dr. Adam Booth Word Count 14744 words
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ABSTRACTIn the Pagosa Springs, Colorado USA, students of Imperial College London and ColoradoSchool of Mines Geophysics Camp 2012 have performed geophysical analyses. Seismicdata, comprising P-wave and S-wave data acquired along two lines (North Line and ZenGarden), were interpreted to analyse near surface geology for geotechnical and groundwaterpurposes.Refraction analyses were performed using gradient-intercept, reciprocal, time term inversionand tomographic inversion methods to calculate the velocity and thickness of eachsubsurface layer. The presence of significant refractor overlaps favoured reciprocal and timeterm inversion methods as it allowed enough room for delay time window analysis to beperformed.Results of each of these methods show a strong correlation in velocity and thickness values.Output of the time term inversion was fed into the tomographic inversion as a starting model.Convergence to a local minimum was reached after about 10 iterations, with an RMS error ofless than 10% in most cases.Analyses of the results in the North Line and Zen Garden area show a slightly undulatingthree layer near surface geology with a dip. Unconsolidated sediments with depth of about 2m and properties that are consistent with shale were interpreted. The layer occupying adepth between 2 m to around 15 m was interpreted to be water saturated sandstone. Thedepth over 15 m seems like sandstone. However because the depth over 15 m is notreachable with ray tracing path, it is not possible to sample beyond ~15 m with the hammerseismic data.By using the velocities acquired from tomographic inversion, datum static correction(including refraction static correction) has been performed to the reflection data, after stackand show improvement in terms of continuity of reflectivity. However, it suffers frominsensitivity due to its very shallow features.Junghee Kim 1
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Table of ContentsABSTRACT............................................................................................................................................ 1ACKNOWLEDGEMENT ...................................................................................................................... 9CHAPTER ONE ................................................................................................................................... 101.0 Introduction ............................................................................................................................... 101.1 Objectives ....................................................................................................................................... 11CHAPTER TWO .................................................................................................................................. 122.0 Geological setting of Pagosa Springs, Colorado USA .............................................................. 12CHAPTER THREE .............................................................................................................................. 153.0 Theory and Literature review .................................................................................................... 153.1 Refraction Seismic Method ....................................................................................................... 153.2 Time-Distance curves for layered media .................................................................................. 163.3 Hidden Layers, Velocity Inversions, and Blind Zones ............................................................. 203.4 Refraction Arrival picking and time adjustments ..................................................................... 223.5 Manual picking and automatic picking of traveltimes .............................................................. 223.6 Reciprocal Time Correlation ..................................................................................................... 233.7 Refraction Interpretation ........................................................................................................... 243.8 Gradient-Intercept method ........................................................................................................ 243.9 Delay-Time Concept ................................................................................................................. 243.10 Reciprocal Method ........................................................................................................................ 263.11 Term-time inversion.................................................................................................................. 313.12 Tomographic inversion method .................................................................................................... 35CHAPTER FOUR................................................................................................................................. 394.0 METHODOLOGY ................................................................................................................... 394.1 Data acquisition ........................................................................................................................ 404.3 Refraction Data Analysis .......................................................................................................... 46 4.3.1 Basic refraction analysis in North Line.................................................................................... 46 4.3.1.1 Promax .................................................................................................................................. 46 4.3.1.2 Geometry assignment............................................................................................................ 46 4.3.1.3 Initial data analysis and quality control ................................................................................ 47 4.3.1.4 First Break Picking in Promax .............................................................................................. 47 4.3.1.5 Extraction to Matlab ............................................................................................................. 48 4.3.1.6 Gradient intercept method ..................................................................................................... 49Junghee Kim 2
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4.3.2 Advanced refraction analysis (North Line) ............................................................................ 50 4.4.2.1 Seisimager ............................................................................................................................. 50 4.4.2.2 Initial data analysis and quality control ................................................................................ 50 4.4.2.3 Data Processing ..................................................................................................................... 50 4.4.2.4 Elevation importing. ............................................................................................................. 50 4.4.2.5 Amplitude Recovery ............................................................................................................. 51 4.4.2.6 Travel Time Pick and QC ..................................................................................................... 52 4.4.2.7 Reciprocal Time Check......................................................................................................... 52 4.4.2.8 First break picks of P-wave in North Line ............................................................................ 53 4.4.2.9 Advanced Seismic Refraction Analysis using Seisimager .................................................... 53 4.4.2.10 Layer assignment ................................................................................................................ 53 4.4.2.11 Reciprocal method .............................................................................................................. 54 4.4.2.12 Time term inversion ............................................................................................................ 55 4.4.2.13 Tomographic inversion ....................................................................................................... 56 4.3.3 Seismic Reflection Data Processing and Analysis in North Line ............................................ 60 4.3.3.1 Refraction Muting ................................................................................................................. 60 4.3.3.2 Bandpass Filtering ................................................................................................................ 62 4.3.3.3 Static Correction ................................................................................................................... 64 4.3.3.3.1 Elevation Statics Analysis in North line. ........................................................................... 65 4.3.3.3.2 Datum static correction from tomographic inversion of Seisimager in Promax: ............... 66 4.3.3.4 Stacking................................................................................................................................. 68 4.3.4 Comparison with the other methods (DC-resistivity) .............................................................. 71 4.3.4.1 DC Resistivity Survey........................................................................................................... 71 4.3.5 Advanced refraction analysis (Zen Garden ) ........................................................................... 73 4.3.5.1 First break picks of P-wave in Zen Garden........................................................................... 73 4.3.5.2 S-wave first break picking .................................................................................................... 74 4.3.5.3 Time-term inversion and Tomographic inversion in Zen Garden......................................... 76 4.3.6 Comparison with Ground Penetration Radar (GPR) ............................................................... 77 4.3.6.1 GPR (Ground Penetration Radar) ......................................................................................... 77CHAPTER FIVE. ................................................................................................................................. 825.0 RESULTS AND DISCUSSION ............................................................................................... 825.1 Basic refraction analysis in North Line........................................................................................... 82 5.1.1Results from Gradient-Intercept method on the North line ...................................................... 82Junghee Kim 3
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5.2 Advanced seismic refraction analysis in North Line ...................................................................... 86 5.2.1. Time Term Inversion .............................................................................................................. 86 5.2.2 Tomographic Inversion ............................................................................................................ 89 5.2.3 Reciprocal Method ................................................................................................................... 995.3 Statics analysis of P-wave data in North Line .............................................................................. 103 5.5.1 Elevation static correction from first break picks picked in Promax: .................................... 103 5.5.2 Datum statics from tomographic inversion . .......................................................................... 104 5.5.3 Application of static correction to the stack ........................................................................... 106 5.5.4 Comparison of the stack with results from refraction analysis. ............................................. 109 ........................................................................................................................................................ 111 5.5.5 Comparison with the result of DC-resistivity survey in North line area. ............................... 1125.4 Advanced refraction analysis in Zen Garden ................................................................................ 114 5.4.1 P-wave velocity model analysis in Zen Garden ..................................................................... 114 5.4.2 S-wave Velocity model from tomographic inversion in Zen Garden .................................... 119 5.4.3 Poison’s ratio analysis............................................................................................................ 121 5.4.4 Vp/Vs analysis ....................................................................................................................... 123CHAPTER SIX. .................................................................................................................................. 1256.0 Conclusions and Recommendations ....................................................................................... 125References ........................................................................................................................................... 127Appendix ............................................................................................................................................. 130List of tablesTable 4-1 Summary of data acquisition in Pagosa Springs Colorado USA ........................................................... 42Table 5-1 Depth model from basic refraction analysis ........................................................................................ 85Table 5-2 Velocity model from basic refraction analysis ..................................................................................... 85Table 5-3 Seismic Velocities of Earth Materials (Gary Mavko, 2005) .................................................................. 99Table 5-4 P- to S-wave velocity and Poisson’s ratios calculated from P- and S-wave in Zen Garden ................ 121Junghee Kim 4
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List of figuresFigure 1-1Seismic waves and the behaviour at interfaces .................................................................................... 10Figure 2-1 Location of Pagosa Springs in entire map of United States of America. (Map is copyright GoogleEarth) .................................................................................................................................................................... 13Figure 2-2 Areal Map of the Structures in the San Juan Basin with the area of Pagosa Springs outlined in red (Imperial College London and Colorado School of Mines Students of the geophysics field camp, 2012) .............. 13Figure 2-3 Areal map with the Archuleta anticlinorium showing relations with the San Juan Basin and otherbasin. ( Imperial College London and Colorado School of Mines Students of the geophysics field camp, 2012) .. 14Figure 3-1 Relationship between the angles of incidence and refraction ............................................................. 15Figure 3-2 Source-to-receiver raypath of a refracted ray in a two-layer case. ..................................................... 16Figure 3-3 Traveltime-offset curve for a horizontal interface two-layer case ...................................................... 17Figure 3-4 Source-to-receiver raypath of a refracted ray in a three-layer horizontal case ................................... 18Figure 3-5 Traveltime-offset curve for a horizontal interface three-layer case .................................................... 20Figure 3-6 Hidden layer problem in refraction caused by a layer having insufficient thickness and velocitycontrast................................................................................................................................................................. 21Figure 3-7 Blind layer problem in refraction caused mainly by a velocity inversion. ............................................ 22Figure 3-8 Refraction picking options: t0 is the first break (first kick) time, t1 is the first arrival time through thefirst inflection time, and t2 to t7 are the trough, zero crossing, and peak times (Cox, 1999) .............................. 23Figure 3-9 Principle of the delay-time method ..................................................................................................... 25Figure 3-10 Principle of reciprocal method ........................................................................................................... 26Figure 3-11 Principle of reduced traveltimes ........................................................................................................ 28Figure 3-12 Principle of time-term inversion (in case that the refractor is parallel to the ground surface) ......... 31Figure 3-13 Principle of time-term inversion (in case that the refractor is non-parallel to the ground surface) .. 33Figure 3-14 Process of depth calculation in time-term inversion.......................................................................... 34Figure 3-15 Principle of tomographic inversion .................................................................................................... 35Figure 4-1 Project work-flow ................................................................................................................................ 39Figure 4-2 Data Acquisition work-flow ................................................................................................................. 40Figure 4-3 hammer seismic showing different p-wave ray paths ......................................................................... 41Figure 4-4 Data acquisitions of P-wave and S-wave ............................................................................................. 41Figure 4-5 Elevation profile of survey area (North line) (information from GPS in Colorado field camp) ............. 43Figure 4-6 Data conversion work-flow .................................................................................................................. 43Figure 4-7 General Cross-section of Pagosa Springs showing location of North line and Zen Garden withexaggerated vertical scale in larger detail. ........................................................................................................... 44Figure 4-8 map of survey area (Map is copyright Google Earth) ......................................................................... 45Figure 4-9 work-flow of basic refraction analysis in North Line........................................................................... 46Figure 4-10 Geometry assignment screen of Common Depth Point (CDP) and Fold in Promax ........................... 47Figure 4-11 Deciding what pick to make for the first arrivals, First Kick, Trough or Peak. ................................... 48Figure 4-12 First break picking on first-kick in Promax ......................................................................................... 48Figure 4-13 Gradient-intercept method graph ..................................................................................................... 49Figure 4-14 work-flow of advanced refraction analysis in North Line .................................................................. 50Figure 4-15 Original data before applying any form of gain. ............................................................................... 51Figure 4-16 Data in figure 4-15 after amplitude correction, stretching. .............................................................. 51Figure 4-17 Reciprocal test for two shots with significant refractor overlap. ....................................................... 52Figure 4-18 Example of P-wave first break picking on first-kick ........................................................................... 53Figure 4-19 Example of layer assignment ............................................................................................................. 54Junghee Kim 5
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Figure 4-20 Example of reverse line forming with delay time line for reciprocal method .................................... 55Figure 4-21 Example of Layered model from time-term inversion ....................................................................... 56Figure 4-22 Process of Tomographic inversion ..................................................................................................... 57Figure 4-23 Design of the number of layers for initial model ............................................................................... 58Figure 4-24 Ray tracing path in tomographic inversion ....................................................................................... 59Figure 4-25 work-flow of seismic reflection data processing and analysis in North Line ..................................... 60Figure 4-26 Refraction muting in Promax. (left: before refraction muting, middle: applying refraction muting,right : after refraction muting ) ............................................................................................................................ 61Figure 4-27 Aliased reflectors of data in FK spectrum analysis ............................................................................ 62Figure 4-28 Schematic drawing on cut range of Bandpass (frequency: 50 – 100 - 200 - 400 Hz)......................... 63Figure 4-29 Bandpass filter application ( left: gather before applying bandpass, right: gather after applyingbandpass............................................................................................................................................................... 64Figure 4-30 schematic geometry for elevation statics with data from first break picks on first-kick of Promax .. 65Figure 4-31 schematic geometry for datum statics using data from tomographic inversion of Seisimager ........ 66Figure 4-32 Screen showing difficulties on velocity picking in Promax ................................................................. 68Figure 4-33 Schematic drawing showing possibility of use of constant velocity for Normal Move Out in shortoffset ..................................................................................................................................................................... 69Figure 4-34 Expected reflector through a look into gather in Promax ................................................................. 70Figure 4-35 Reflector shown in Brute stack in Promax ......................................................................................... 70Figure 4-36 work-flow of comparison of North Line with DC-resistivity ............................................................... 71Figure 4-37 SP and inverted resistivity profiles of PAGO 02 (Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 72Figure 4-38 North line area where North line hammer seismic survey line crossing with PAGO 02 DC resistivitysurvey line (Map is copyright Google Earth) ......................................................................................................... 72Figure 4-39 Work-flow of advanced refraction analysis in Zen Garden ................................................................ 73Figure 4-40 Example of P-wave firstbreak picking on first-kick in Zen Garden ..................................................... 74Figure 4-41 Example of the raw data of S-wave in Zen Garden ........................................................................... 75Figure 4-42 Example of choosing bad trace of S-wave in Zen Garden .................................................................. 75Figure 4-43 Example of S-wave firstbreak picking on first-kick in Zen Garden ..................................................... 76Figure 4-44 Work-flow of comparison of Zen Garden with GPR ........................................................................... 77Figure 4-45 GPR acquisition comprising of the radar components and the analogue interpretation of a radartime section. Tx: Transmitter, Rx: Receiver (Redrawn from Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 78Figure 4-46 Barn 3 survey line ( red line: SW- NE ) cited from Google Map ........................................................ 79Figure 4-47 General cross-section of Pagosa Springs showing the location of Barn 3 and Zen Garden. Verticalscale has been exaggerated to show features in larger detail. (Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 80Figure 4-48 General cross-section of the location of GPR acquisition in data acquisition line of Barn 3. Verticalscale has been exaggerated to show features in larger detail. (Imperial College London and Colorado School ofMines Geophysics Field Camp, 2012).................................................................................................................... 81Figure 5-1 Depth model generated from picking firstbreak on the first pick in Promax ...................................... 82Figure 5-2 Depth model generated from picking firstbreak on first kick in Promax ............................................ 83Figure 5-3 Depth model generated from picking firstbreak on first trough in Promax ....................................... 83Figure 5-4 Velocity model generated from picking firstbreak on the first pick in Promax ................................... 84Figure 5-5 Velocity model generated from picking firstbreak on first kick in Promax .......................................... 84Figure 5-6 Velocity model generated from picking firstbreak on the first trough in Promax ............................... 85Junghee Kim 6
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Figure 5-7Connected Layer assignment of whole North line in Plotrefa TM of Seisimage ................................... 87Figure 5-8 Layered model from time-term inversion of North line with smoothing effect (Smoothing passes: 3)added in Plotrefa TM of Seisimager...................................................................................................................... 88Figure 5-9 Principle of designing the number of layers for the initial model ........................................................ 89Figure 5-10 the image of one move-up time term inversion result chosen for parameter tests for initial model inNorth line in comparison with the whole North line time term inversion image in Plotrefa TM of Seisimager ... 91Figure 5-11 images of one pattern time term inversion result chosen for parameter tests for initial model inNorth line in Plotrefa TM of Seisimager ( (a) P-wave velocity 30 m/s – 3000 m/s, the number of layers 10 (b)P-wave velocity 30 m/s – 3000 m/s, the number of layers 15 (c) P-wave velocity 30 m/s – 3000 m/s, thenumber of layers 18 ............................................................................................................................................. 92Figure 5-12 images of time term inversion result chosen for parameter tests for initial model in North line inPlotrefa TM of Seisimager ((a) P-wave velocity 30 m/s – 1000 m/s, the number of layers 15 (b) P-wavevelocity 30 m/s – 3000 m/s, the number of layers 15 (c) P-wave velocity 30 m/s – 10000 m/s, the number oflayers 15 ) ............................................................................................................................................................. 93Figure 5-13 The image of initial model in whole North line in Plotrefa TM of Seisimager ( calculated withparameters of P-wave velocity 30 m/s – 3000 m/s, the number of layers 15 ...................................................... 95Figure 5-14 Misfit between synthetic and observed travel time as a function of the iteration number. Observethe lack of significant reduction in the travel time misfit after about 10 iterations. ............................................ 96Figure 5-15 the image of P-wave velocity model from tomographic inversion in whole North line in Plotrefa TMof Seisimager (value 10 was chosen for the number of iteration ) ....................................................................... 97Figure 5-16 the image of Ray tracing path of P-wave velocity model from tomographic inversion in whole Northline in Plotrefa TM of Seisimager .......................................................................................................................... 98Figure 5-17 an image of reciprocal method showing delay time line and reverse time line in one move-up ofNorth line in Plotrefa TM of Seisimager (delay times in both sides are calculated and averaged ) ................... 100Figure 5-18 the image of P-wave velocity model generated by reciprocal method in one move-up of North linein Plotrefa TM of Seisimager ( delay times in both sides are calculated and averaged ) ................................... 101Figure 5-19 Comparison between images of P-wave velocity models generated by reciprocal method and time-term inversion in one move-up of North line in Plotrefa TM of Seisimager (Note that both methods areconducted in same position) ............................................................................................................................... 102Figure 5-20 plots of Elevation static correction on P-wave obtained from first break pick on first kick inNorthline of receiver shown in Promax . ............................................................................................................. 103Figure 5-21 plots of Elevation static correction on P-wave obtained from first break pick on first kick in Northlineof source shown in Promax . ............................................................................................................................... 103Figure 5-22 Values of LVL Static ( refraction static), Elevation static correction of receiver and total datum staticcorrection shown in Promax . The values of elevation static correction and LVL static correction are added up tofind datum static correction. .............................................................................................................................. 104Figure 5-23 plots of Datum static correction on P-wave obtained from tomographic inversion in North line ofreceiver shown in Promax . ................................................................................................................................. 105Figure 5-24 plots of Datum static correction on P-wave obtained from tomographic inversion in North line ofsource shown in Promax . ................................................................................................................................... 105Figure 5-25 the image of stack not applied with static correction (only bandpass applied : Bandpass frequencyrange : 50 – 100 -200 -400 hz . ........................................................................................................................... 106Figure 5-26 the image of stack applied with elevation static correction ( bandpass and elevation staticcorrection applied : Bandpass frequency range : 50 – 100 -200 -400 hz .) ......................................................... 107Figure 5-27 the image of stack applied with Datum static correction ( bandpass and datum static correctionapplied applied : Bandpass frequency range : 50 – 100 -200 -400 hz Here Datum static correction = LVL staticJunghee Kim 7
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correction ( Refraction static correction (LVL) .................................................................................................... 107Figure 5-28 the image of stack ( only bandpass applied : Bandpass frequency range : 50 – 100 -200 -400 hz . 108Figure 5-29 the image of stack applied with elevation static correction ( bandpass and elevation staticcorrection applied : Bandpass frequency range : 50 – 100 -200 -400 hz .) ......................................................... 108Figure 5-30 the image of stack applied with datum static correction ( bandpass and datum static correctionapplied applied : Bandpass frequency range : 50 – 100 -200 -400 hz Here datum static correction = LVL staticcorrection ( Refraction static correction )+ elevation static correction............................................................... 108Figure 5-31 A possible fault by comparison between refraction processed image and reflection processed imagein North line. (a) image from time-term inversion (b) image from tomographic inversion (c) image from brutestack applied with datum statics correction. ...................................................................................................... 110Figure 5-32 A possible fault (F1) by comparison of the stack with superimposed and flattened refractionprocessed image( from tomographic inversion) in North line ............................................................................ 111Figure 5-33 A possible fault expected by result from DC-resistivity survey and Hammer seismic survey in NorthLine area (The DC-resistivity model is fit to the PAGO02 pararelly, and the tomographic inversion image is fit tothe North line in parallel) DC-resistivity image is cited from Imperial College London and Colorado School ofMines Geophysics Camp 2012. ........................................................................................................................... 113Figure 5-34 the image of P-wave velocity model generated by time term inversion in Zen Garden in Plotrefa TMof Seisimager ...................................................................................................................................................... 114Figure 5-35 the image of P-wave velocity model from tomographic inversion in Zen Garden in Plotrefa TM ofSeisimager (value 10 was chosen for the number of iteration ) ......................................................................... 115Figure 5-36 the image of Ray tracing path of P-wave velocity model from tomographic inversion in Zen Gardenin Plotrefa TM of Seisimager .............................................................................................................................. 116Figure 5-37 Comparison of P-wave velocity model from tomographic inversion and subsurface model from basicgradient intercept method done by Imperial College London and Colorado School of Mines Geophysics FieldCamp, 2012 ( right Figure.- cited from Imperial College London and Colorado School of Mines Geophysics Camp,2012 (right Figure cited from Imperial College London and Colorado School of Mines Geophysics Camp, 2012)............................................................................................................................................................................. 117Figure 5-38 the image of S-wave velocity model generated by time term inversion in Zen Garden in Plotrefa TMof Seisimager ...................................................................................................................................................... 118Figure 5-39 the image of S-wave velocity model from tomographic inversion in Zen Garden in Plotrefa TM ofSeisimager (value 10 was chosen for the number of iteration) .......................................................................... 118Figure 5-40 the image of Ray tracing path of S-wave velocity model from tomographic inversion in Zen Gardenin Plotrefa TM of Seisimager .............................................................................................................................. 119Figure 5-41 Comparison of shapes of P-wave data and S-wave data ................................................................ 120Figure 5-42 Chart of Poisson’s ratio, Vp/Vs ratio and P-wave velocity (Redrawn from Thomas M. Brocher, 2005)............................................................................................................................................................................ 122Figure 5-43 Chart of Vp, Vp/Vs ratio and Porosity in Zen Garden ( Redrawn from E.R.(Ross) Grain, 2000) .... 123Figure 5-44 (a) Seismic section at Barn 3 and (b) its interpretation related to the Dakota Sandstone. (ImperialCollege London and Colorado School of Mines Geophysics Field Camp, 2012) .................................................. 124Junghee Kim 8
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ACKNOWLEDGEMENTDr. Adam Booth. I would like to express my special appreciation to him. He is my supervisor.Without his guidance and supervision, the completion of this project would not be possible.In addition, I would like to express special gratitude to Professor Helmut for his kind supportsand guidance throughout this entire course.I also appreciate Faculty of Colorado School of Mines for the efforts that are made to acquirethese data from Pagosa Springs, Colorado, USA.Sincere thanks to Mr Seth who was in charge of data acquisition in Pagosa Springs for hiskind support and guidance.Special thanks to My sister, Mrs. In-hee Kim and his husband Mr. Isaac Choi, my parent,Mrs. Sun-hee Kim, Mr. Hyun-dong Kim.And I also thank Kenneth for his spiritual supports.Junghee Kim 9
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CHAPTER ONE 1.0 IntroductionSeismic surveys measure the earth’s elastic properties using seismic waves (Sheriff 2002).The source of these disturbances can be controlled as in the case of exploration andengineering seismology, or it can be uncontrolled as in the case of earthquake seismology.(Dobrin 1976) The propagation is described by the elastic wave equation, which is derivedfrom two laws of physics, Hooke’s law and Newton’s second law of motion. (Dobrin 1976)When an elastic wave propagates through a medium in the earth is reflected, refracted andtransmitted at an interface (Figure 1-1) (Dobrin 1976). The wave can also be diffractedaround discontinuities. (Dobrin 1976) Figure 1-1 Seismic waves and the behaviour at interfaces (Dobrin 1976; Waters 1997)There are two forms of seismology, reflection and refraction seismology (Jakubowicz 2012).Refraction seismology involves the recording, processing and analysis of refracted seismicenergy and is mainly used for near surface studies. Reflection seismology involvesprocessing and analysing seismic reflected energy. Reflection surveys are mainly applied inexploration for mining and hydrocarbon exploration (Dobrin 1976), and crustal studies(Reading et al, 2011). Seismic experiments performed for near surface investigations arereferred as shallow seismic surveys. (Karastathis et al. 2007)Shallow seismic studies are often applied to detect geologic structures in fault zones and tofind shallow, soft layers of underground earth materials especially in area of rapidJunghee Kim 10
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urbanisation and heavy agriculture. (Karastathis et al. 2007)Seismic refraction survey using a Hammer source was conducted along selected line acrossPagosa Springs, Colorado in June 2012. The aim was to perform near surface study andcharacterisation of the hydrothermal activities in the area. Although Pagosa Springs inColorado is famous for the hydrothermal activities, these are still not well understood.(Imperial College London and Colorado School of Mines Geophysics Field Camp 2012)In this project, near surface study and characterisation using refraction analysis of dataacquired at Pagosa Springs will be performed with a view to determining the depth of thebedrock and the ground water, the lateral and vertical changes in lithology, the lithology typeand investigating the structural features such as micro faults.1.1 ObjectivesThe aims of the near surface study in Pagosa Springs are as follows: To use P-wave and S-wave refraction methods to obtain velocity-depth models for near-surface layering at Pagosa Springs. To combine P- and S-wave observations to quantify physical properties of near- surface layering, and to propose lithology. To investigate the interpretation of P-wave refraction data as a reflection profile, including a near-surfaceJunghee Kim 11
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CHAPTER TWO 2.0 Geological setting of Pagosa Springs, Colorado USAPagosa Springs is located on the northeast edge of the San Juan Basin as seen in Figure 2-2. ( Imperial College London and Colorado School of Mines, geophysics filed camp 2012)This is a large depositional basin concentrated in western New Mexico and Four Cornersregion of the western United States (Fred 1982).The basin is bordered in the north by theSan Juan Mountains of southern Colorado, in the northeast by the Chama Basin, in the eastby the Nacimiento and San Pedro Uplifts, in the south by the Zuni Uplift and the ZuniMountains of New Mexico and in the west by the Defiance Uplift of eastern Arizona andwestern New Mexico. The central basin with deepest sedimentary units is mainly located innorth western New Mexico and a small part of southern Colorado. (Fred 1982) Uplift ofmountain ranges almost prior to the Cambrian age and the transgression of multipleseaways beginning in the late Cambrian age caused this basin to form. This is the reasonwhy the basin includes almost continuous column of sedimentary units beginning in the lateCambrian and continuing until the glaciations and orogenies of the late Cenezoic. (Fred1982). On the Archuleta anticlinorium, Pagosa Springs is located in the northeast edge ofthis basin. (Fred 1982) The Archuleta anticlinorium is located in the edge of the San JuanBasin starting from southern Colorado with a north- northwest trend, continuing into northcentral Arizona. (Fred 1982) The region is located 15 miles west of the continental dividewith the San Juan River serving as the primary stream system because it flows from theDivide to the Pacific Ocean to the Southwest. Its allochthonous folding over the underlyingbasement is the most significant characteristics of this structure. (Fred 1982) A shallownorth-north western trending anticline through Pagosa Springs is produced by this. Thisgives the 12000 ft of sedimentary units in the area, a dip of about 5-10˚ towards the SanJuan Mountains in the north eastern half of the anticlinorium and a similar dip towards thebasin on the south western half. (Fred 1982) To the north, the units merge with thesurrounding basins beneath the San Juan Mountains. (Fred 1982) However, to the south,the units increase in dip when they move towards the main basin. (Fred 1982) In the Pagosa Springs, Colorado USA, geophysical analyses have been performed bystudents of Imperial College London and Colorado School of Mines during the geophysicalsummer camp 2012. Different geophysical experiments were performed in this area. One ofsuch was the refraction seismic method which is to analyse near surface geology of the areafor geotechnical and groundwater purposes.Junghee Kim 12
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Figure 2-1 Location of Pagosa Springs in entire map of United States of America. (Map is copyright Google Earth) Figure 2-2 Areal Map of the Structures in the San Juan Basin with the area of Pagosa Springs outlined in red ( Imperial College London and Colorado School of Mines geophysics field camp 2012)Junghee Kim 13
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Figure 2-3Areal map with the Archuleta anticlinorium showing relations with the San Juan Basin and other basin. (Imperial College London and Colorado School of Mines, geophysics field camp 2012)Junghee Kim 14
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CHAPTER THREE 3.0 Theory and Literature review3.1 Refraction Seismic MethodRefraction can be defined in terms of the change in direction of a seismic ray or wavefront atan interface between layers of different velocities (Cox 1999). The relationship between theangles of incidence and refraction at the interface (Figure 3-1) is governed by Snell’s law,which is given as (Craig Lippus 2007): (2.1)Where , represent the angles of incidence and refraction and , represent thevelocities in the first and second layer respectively. (Craig Lippus 2007) Figure 3-1Relationship between the angles of incidence and refraction (Jacob Fokkema and Nafi Toksoz 2012)When the angle of incidence is such that the refracted wavefront is perpendicular to theinterface ( ), it is referred to as critical angle of incidence ( ) and the refracted ray travelsalong the interface between the two layers. Equation (2.1) is the then adjusted to the form(Craig Lippus 2007):: (2.2)Junghee Kim 15
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The waves that travel to and along the interface between the two layers and return to thesurface through the upper layer are referred to as refraction waves, head waves, Mintropwaves, or bow waves (Cox 1999).3.2 Time-Distance curves for layered mediaFigure 2.5 shows the raypath of a refracted ray from a source location at S to a receiverlocation at R for a two-layer horizontal interface case. The total traveltime ( ) for thisraypath, having a source-to-receiver separation of x is given as the sum of the traveltime oneach of the three sections making up the path. (Jacob Fokkema and Nafi Toksoz 2012) i.e: (2.3)This implies that:Rearranging the equation: (2.4)Figure 3-2 Source-to-receiver raypath of a refracted ray in a two-layer case (Jacob Fokkema and Nafi Toksoz 2012).Using Snell’s law (Jacob Fokkema and Nafi Toksoz 2012)Junghee Kim 16
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(2.5)Finally we have: (2.6)Equation (2.5) represents a straight line with a slope of and an intercept of given by: (2.7)Figure 2.5 shows the traveltime graph representing the propagation of the refracted ray for atwo-layer horizontal case. From the graph we can calculate and use it to estimate to therefractor z. (Jacob Fokkema and Nafi Toksoz 2012)Figure 3-3Traveltime-offset curve for a horizontal interface two-layer case (Jacob Fokkema and Nafi Toksoz 2012)From Equation (2.7), we have that (Jacob Fokkema and Nafi Toksoz 2012):Junghee Kim 17
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(2.8)Using equation 2.2 and some trigonometric properties, we have that (Jacob Fokkema andNafi Toksoz 2012) : (2.9)Figure 3-4 Source-to-receiver raypath of a refracted ray in a three-layer horizontal case (Jacob Fokkema and NafiToksoz 2012)For a three-layer case having a raypath diagram shown in figure 3-4, Equations (2.5 – 2.7)can be extended following the same processes as above to yield the total traveltime asJunghee Kim 18
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(Jacob Fokkema and Nafi Toksoz 2012), (2.10)This again is a straight line equation with a slope of and an intercept of given as: (2.11)The depth of the first layer is calculated as before, while the thickness of the second layer isgiven as: (2.12)Therefore, (2.13)Junghee Kim 19
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Figure 3-5 Traveltime-offset curve for a horizontal interface three-layer case (Jacob Fokkema and Nafi Toksoz 2012)Figure 3-5 shows the traveltime curve for the three layer case from which we read theintercept times and calculate the thicknesses of the various interfaces.For a multilayer problem, Equation (2.14) is given by (Cox 2009) (2.14)Where (2.15)3.3 Hidden Layers, Velocity Inversions, and Blind ZonesIn order to be detected in a first arrival refraction survey, a layer must satisfy two conditions:(a) be underlain by a layer of higher velocity so that head waves are produced, and (b) havea thickness and velocity such that the head waves become first arrivals at some range(Kearey and Brooks, 2002). It is possible for layers to exist in the Earth, yet not produce anyrefracted first-arrival waves, and a simple first arrival refraction survey will not be able toJunghee Kim 20
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detect these layers if these conditions are not met. The possibility of undetected layersshould therefore be considered when interpreting refraction data. (Philip Kearey et al. 2002)Figure 3-6 Hidden layer problem in refraction caused by a layer having insufficient thickness and velocity contrast(Philip Kearey et al. 2002).In practice, two different types of problem are shown: (1) Hidden layer, and (2) Blind zone.A hidden layer, from its name, is one that cannot be detected by first arrival seismicrefraction method, and may be caused by insufficient thickness and velocity contrast of thelayer (Cox, 1999). The layer produces head waves, but does not give rise to first arrivals(Kearey and Brooks, 2002). Rays travelling to deeper levels arrive before those criticallyrefracted at the top of the layer in question (Figure 3-6). In such a case, a method of surveyinvolving recognition of only first arrivals will fail to detect the layer. It is good practice toexamine the seismic traces for possible arrivals occurring behind the first arrivals. (PhilipKearey et al. 2002)A blind layer violates the first condition necessary for first arrival refraction experimentdetection by resulting from a low-velocity layer, as illustrated in Figure 3-7 (Kearey andBrooks 2002). Rays are critically refracted at the top of such a layer and the layer willtherefore not give rise to head waves. The interpretation of travel-time curves, in thepresence of a low-velocity layer, leads to an overestimation of the depth to underlyinginterfaces. (Philip Kearey et al. 2002)Junghee Kim 21
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Figure 3-7 Blind layer problem in refraction caused mainly by a velocity inversion (Philip Kearey et al. 2002).3.4 Refraction Arrival picking and time adjustmentsThe first step in the interpretation of a refraction experiment data is to review and pick thearrival times (Cox 1999). While the review phase involves the initially analysis of the data tobe picked, the picking phase is concerned with the actual picking of traveltimes, which isusually done either manually or automatically. Certain adjustments of reciprocal time arealso performed on the picked traveltimes before any form of interpretation is then carried out.(Cox 1999)3.5 Manual picking and automatic picking of traveltimesFigure 2.10 shows a refraction arrival in which the various forms of picks (from first kick,peak, trough) has been shown. Picking requires that we have a broadband signal, minimalfiltering of data, a good signal-to-noise ratio, and a high gain display (Cox 1999). First breakor kick (represented by t0 in Figure 3-8 ) is usually picked because a change in frequencywith offsets, receiver and source locations (usually common with land surveys) may cause ashift relative to the first break. (Cox 1999)Junghee Kim 22
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Figure 3-8 Refraction picking options: t0 is the first break (first kick) time, t1 is the first arrival time through the firstinflection time, and t2 to t7 are the trough, zero crossing, and peak times (Cox 1999)In most settings, it is desirable in manual picking of travels times that the accuracy stayswithin 1 or 2 ms for individual picks (Cox 1999).In the presence of a large dataset the picking is usually automated. Automated picking workswell in a good signal-to-noise dataset, and the first arrivals are well defined. (Cox 1999)3.6 Reciprocal Time CorrelationRegardless of the subsurface structure, seismic reciprocity condition between any two pointsmust be satisfied for the surface-consistent refracted travel times,(Hagedoorn 2006) i.e.: (2.16)This condition should be tested and corrected prior to performing any form of interpretation.It is usually done by calculating the reciprocal time misfits between all pairs of shot locations(Si and Sj) with reciprocal (reversed) recording (Hagedoorn 2006): (2.17)When the misfit ( ) is large, corrections are then applied to traveltime picks, though it isadvised that the picking be redone when possible (Hagedoorn 2006).Junghee Kim 23
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3.7 Refraction InterpretationIn an area with simple planar refractors and the velocities in the overlying layers are laterallyinvariant, any of Equations (2.4) to (2.17) can be used to determine the layer velocities andtheir corresponding depths. However, in practice the geology is usually very complex andspecial efforts are therefore required in refining these equations and in applying themsubsequently (Jacob Fokkema and Nafi Toksoz 2012).Refraction interpretation methods are broadly divided into two approaches (Cox 1999):Those in which the data are analysed at a common surface location and those in which thedata are analysed at a common subsurface location.Inversion can also be used to interpret refraction data. Tomographic and time-terminversions are the most common applied in practice.3.8 Gradient-Intercept methodThe gradient-intercept method (also called intercept method) is used as an interpretationmethod when the geology is simple and planar. It uses the Equations derived above ((2.4) ~2.17)), where the intercept time (zero offset time) is used to determine the refractor depth atthe source location (Jacob Fokkema and Nafi Toksoz 2012). (Figure 3-2).3.9 Delay-Time ConceptIn a complex subsurface where the interfaces are undulating and multi-layered, most of therefraction-statics methods, such as the Plus-Minus and the Generalized Reciprocal methodsare based on the delay-time approximation of refracted travel times (Hagedoorn 2006) tosolve for the refraction statics. Consider a source located at point S and a receiver at pointR at the surface (Figure 2.4). In the delay-time approximation, the refractor is considered asnear-horizontal between the two points, and the distance between them is much greater thanthe critical distance. (here, critical distance means the minimum distance from the energysource at which the first critical refraction can be received (Jacob T. Fokkema and M.NafiToksoz 2012). Generally, this implies that the velocity of the refractor (bedrock) is muchlarger than that of the overburden.Under these approximations, the travel-time from S to R can then be separated to thesource-side and receiver-side times (Jacob Fokkema and Nafi Toksoz 2012).: (2.18)Junghee Kim 24
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Figure 3-9 Principle of the delay-time method (Jacob Fokkema and Nafi Toksoz 2012).Time can be represented as a sum of the travel time along the reflector and the “sourcedelay” time (Jacob Fokkema and Nafi Toksoz 2012).: (2.19)For source delay, , we therefore have (Jacob Fokkema and Nafi Toksoz 2012): (2.20)In a similar way, the receiver delay time is defined, and the total time from the source to thereceiver is (Jacob Fokkema and Nafi Toksoz 2012) : (2.21)This equation relates the velocity of the bedrock and the depth of the weathering layer to thefirst-arrival travel times. This equation is further inverted to solve for the depths of theweathering layer near the sources and receivers, and the velocity of the refractor (JacobFokkema and Nafi Toksoz 2012).Junghee Kim 25
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3.10 Reciprocal MethodConcept of Delay time in Reciprocal Method is as Figure 3-10. Figure 3-10 Principle of reciprocal method (Jacob Fokkema and Nafi Toksoz 2012).Referring Equation (2.19) and Equation (2.20), if AC = BD, in this case, × 2 (becausein both sides) + (here x = ) (Jacob Fokkema and Nafi Toksoz 2012; Seisimager/2DManual 2005).. (2.22)But if it is different values, Then, (2.23) Similarly, (2.24)AndJunghee Kim 26
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(2.25)Delay time to in Reciprocal method (2.26)If substituting, (2.27) This is equal to, (2.28)In the Figure 3-10, (2.29)Junghee Kim 27
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Therefore, (2.30)Here, to is twice the time required for the seismic energy to travel from P to P’.Delay time DT at point P is defined as below (Jacob Fokkema and Nafi Toksoz 2012;Seisimager/2D Manual 2005).. . (2.31)Computation of reduced traveltime allows us to remove the effect of changing layerthickness on the traveltim curve and give a better measurement of velocity. The delay timeand refractor depth are calculated (Jacob Fokkema and Nafi Toksoz 2012; Seisimager/2DManual 2005).. . Figure 3-11 Principle of reduced traveltime (Jacob Fokkema and Nafi Toksoz 2012; Seisimager/2D Manual 2005)The reduced traveltime at point P for a source at A T’AP (Jacob Fokkema and Nafi Toksoz2012; Seisimager/2D Manual 2005)..Junghee Kim 28
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(2.32)This is same as (2.33)By rearranging, (2.34)Because (2.35) (2.36)Therefore, (2.37)Assuming that the AC = BD, (2.38)Junghee Kim 29
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(2.39)Because (2.40)Therefore, (2.41) (2.42)Therefore, the depth in P point is decided as following (Jacob Fokkema and Nafi Toksoz2012; Seisimager/2D Manual 2005).. (2.43)Note that Equation (2.43) is same as (Jacob Fokkema and Nafi Toksoz 2012;Seisimager/2D Manual 2005). (2.44)Junghee Kim 30
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3.11 Term-time inversionA linear Least-Squares approach is used to define the time-term method. This is todetermine the best discrete-layer solution to the data (Takaya Iwasaki 2002; Seisimager/2DManual 2005).Figure 3-12 Principle of time-term inversion (in case that the refractor is parallel to the ground surface) (TakayaIwasaki, 2002; Seisimager/2D Manual 2005). .Slowness is defined as S which is inverse velocity (Takaya Iwasaki 2002; Seisimager/2DManual 2005). . (2.45) (2.46)Junghee Kim 31
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From Snell’s Law, (2.47)Travel time definition in reciprocal method (in the assumption that the depths in both sidesare same) (2.48)If the total travel time = t from source to receiver, h = z, S1 = 1/V1, S2 = 1/V2 (2.49)C is defined as follows, (2.50)Then (2.51)Z and S2 are not known The example above has assumption that the refractor is parallel to the ground surfaceIf these are non-parallel, curved surfaces, there are three un-knowns Z1, Z3 and S2. (TakayaIwasaki 2002; Seisimager/2D Manual 2005). .Junghee Kim 32
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Figure 3-13 Principle of time-term inversion (in case that the refractor is non-parallel to the ground surface) (TakayaIwasaki 2002; Seisimager/2D Manual 2005). .Now, (2.52)Generalisation, (2.53)In matrix form,Junghee Kim 33
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(2.54)Where m = number of traveltimes, and n = number of receivers (Depths to be calculated).So, Z1, Z2, ••• Zn and S2 are solved. Figure 3-14 Process of depth calculation in time-term inversionTo make it clear, in Figure 3-14, the first source can have many cases of x values withdifferent t values. When the seismic ray is passing P1, many receivers can receive this ray.By the travel times and x values, z1 is decided. The second source does same thing againcalculating z2 and it is repeated up to the last source calculating z3, z4, ··· zn. This is ··,expressed as Equation (2.54).Junghee Kim 34
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3.12 Tomographic inversion methodJacob R. Sheehan et al. (2000) stated that tomographic inversion method is able to resolvevelocity gradients and lateral velocity changes and can be applied in settings whereconventional refraction techniques don’t work. For example, the method can be applied inareas of compaction, karst, and fault zones.Tomographic inversion requires an initial model because this inversion is non-linear problem.Iteratively tracing rays through the model compares the calculated traveltimes to themeasured traveltimes. And it modifies the model and repeats the process until the misfitbetween calculated and measured times is minimised. Therefore, the ultimate goal is to findthe minimum traveltime source and receiver for each source-receiver pair. By solving l(raypath) and s (slowness: inverse velocity). Because both are unknowns, the problem isunder-constrained and an iterative, least-squares approach. (Non-linear problem) (Jacob R.Sheehan et al. 2000 ; Seisimager/2D Manual 2005). Figure 3-15 Principle of tomographic inversion (Jacob R. Sheehan et al. 2000 ; Seisimager/2D Manual 2005). (2.55)S= slowness = velocitylij = raypathJunghee Kim 35
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(2.56)Therefore, (2.57)Or (2.58)Following can be said. ● (2.59) ●This can be expressed asJunghee Kim 36
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(2.60)This is the Least squares method. Generally, M > NThe conditions are required in the tomographic inversion.First, Jacobian matrix requires ray-path.Second, Ray-path cannot be calculated without a velocity model.Third, cannot solve at once.Fourth, must use non-linear Least Square method.Iterative solution of a non-linear Least Squares matrix is as follows. 1) Theoretical value Yo (travel time) for initial value Xo (Slowness) is calculated. (2.61) 2) Calculate residuals (∆Y) between theoretical value Yo and observed value Y. (2.62) 3) Calculate correction value for X(∆Y) by the least squares method (Here, A = raypath) (2.63) 4) Calculate new estimate for X1 ( there X1 = Xo + ∆X ) 5) Put the X1 value back to the model. (2.64)Junghee Kim 37
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This process is repeated until the misfit is close to the minimum.And with the X values (Slowness) and Y values (travel time), the depths of each point aredecided. (Jacob R. Sheehan et al. 2000; Seisimager/2D Manual 2005)In the time-inversion and tomographic inversion, RMS error checking was performed for dataquality purpose.Here Root-mean-square error (2.65)Here n is the number of layer, and Ei is the difference between the inverted and actualvelocities for the ith layer. (Khaled Al Dulaijan 2008)Junghee Kim 38
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CHAPTER FOUR 4.0 METHODOLOGYThis section introduces the source of data acquisition, its preparation technique, dataprocessing and methods of analyses. Procedure of this project is as follows in Figure 4-1. Figure 4-1Project work-flowJunghee Kim 39
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4.1 Data acquisition Figure 4-2 Data Acquisition work-flowThe location of the North line in the Pagosa Springs, 2012 firstly was chosen for survey isbecause according to geological study, this area is assumed to have anomalous featuressuch as fault, and dipping interfaces.(Imperial College London and Colorado School ofMines Geophysics Field Camp 2012) On the location map of the North Line, P-wave seismicrefraction acquisition was performed. Secondly, the location of the Zen Garden was chosenfor survey because this area is very close to North line, the geological feature in this area isassumed to be similar to the North line area. In addition, in the Zen Garden area, S-waveseismic refraction acquisition, as well as P-wave seismic refraction acquisition has beenperformed. The availability of S-wave and P-wave information allow us to calculate Poisson’sratio and Vp/Vs through which the rock properties, lithology, porosity and water spreading inthe area could be analysed.In North Line, shot and receiver spacing were each 3 m, while the shot point was in sameposition of receiver point. In Zen Garden, shot and receiver spacing were each 3m, while theshot point was midway between two adjacent receivers and 24 geophones were deployed ata time in each line making the maximum offset 70.5m. In Gen Garden the shot moves inbetween the geophone spread, down to the end of the line resulting in a total of 24 shots.In North Line, the shot moves in same position of geophone spread, down to the end of theline resulting in a total of 24 shots. Then the setup is rolled along the line until the end of thesurvey line is reached. The experiment was rolled seven times on the North Line, but donejust once on the Zen garden line.P-waves were recorded in both the North line and the Zen garden using vertical geophones,while an addition S-wave survey was carried out in the Zen garden, using horizontalgeophones (Figure 4-4).Junghee Kim 40
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Figure 4-3 hammer seismic showing different p-wave ray paths Figure 4-4 Data acquisitions of P-wave and S-waveA summary of the acquisition set is shown in table 4-1.Junghee Kim 41
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Table 4-1 Summary of data acquisition in Pagosa Springs Colorado USA(Imperial College London and ColoradoSchool of Mines Geophysics Field Camp 2012)Zen Garden area is almost flat (elevation: about 2141 m) and the North line area hastopography as shown in Figure 4.5. (Imperial College London and Colorado School of MinesGeophysics Field Camp 2012) (Appendix. 6)Junghee Kim 42
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Figure 4-5 Elevation profile of survey area (North line) (information from GPS in Colorado field camp) 4.2 Data conversion Figure 4-6 Data conversion work-flowWhen the data were acquired, the file format was SU file. To process the data, the SUformat file had to be converted to SEG-Y file and SEG-2 file.Matlab was used to convert SU format files to SEG-Y for application in Promax for basicanalysis and reflection processing and SEG-2 format files for application in Seisimager foradvanced analysis. ( Mathworks 2012)Promax and Seisimager will be explained later.Junghee Kim 43
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Figure 4-7 General Cross-section of Pagosa Springs showing location of North line and Zen Garden with exaggerated vertical scale in larger detail. ( Imperial College London andColorado School of Mines Geophysics Field Camp 2012 )In Figure 4-7, the blue arrow is directing the locations of North line and Zen Garden. (Imperial College London and Colorado School of MinesGeophysics Field Camp 2012)Junghee Kim 44
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Figure 4-8 map of survey area (Map is copyright Google Earth)Junghee Kim 45
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4.3 Refraction Data AnalysisRefraction analysis basically involves the processing and interpretation of first for variousnear surface parameter estimation.4.3.1 Basic refraction analysis in North Line Figure 4-9 work-flow of basic refraction analysis in North Line 4.3.1.1 PromaxSEG-Y format file is used for this process. With the hammer seismic data in Promax,process of the first break picking is conducted.(Promax 1998) Based on the data obtainedfrom this process, Seismic refraction analysis has been performed further in matlab for thebasic analysis. 4.3.1.2 Geometry assignmentIn this process, geometry information of shot spacing (3 m), receiver spacing (3 m) andmove-ups (patterns)(1 – 24, 25-48, 49 -72, 73 -96, 97 -120, 121 – 144, 145 -168)) have beenassigned.Junghee Kim 46
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Figure 4-10 Geometry assignment screen of Common Depth Point (CDP) and Fold in PromaxThe acquisition was done every move-up (pattern) separately. Once it was done, the linewas rolled up and spread out another line of another pattern. We repeated the process 7times. That is why the fold versus CDP graph looks as 7 peaks.Maximum fold of coverage in Land data (North Line) = The number of channels / (shotinterval/group interval) = 24 / (3/3) = 24 (Jakubowicz 2012) 4.3.1.3 Initial data analysis and quality controlThe original seismic data are initially subjected to quality in other to look for bad shotgathers. The following shot gather were discovered to be really and as such not suitable foranalysis and interpretation. In the initial stage, data were quality controlled for repeatedshots. They were subsequently removed from the dataset. (Appendix 5) 4.3.1.4 First Break Picking in PromaxFirst break picking is to detect or pick the onset arrivals of refracted signals from all thesignals received by the receiver and produced by a source generated. This is sometimescalled first break detection or first arrival picking. (Chugn-Kuang and Chu and Jerry Mendel1994) In this project, first break picking has been done using Promax in each shot.Picking first arrival is faced with the decision of what to pick, First Kick, Peak, or Trough(Figure 3-9).Junghee Kim 47
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Figure 4-11 Deciding what pick to make for the first arrivals, First Kick, Trough or Peak.Picks were made in this project by selecting first kicks first, peak and later trough.In this project, to see the sensitivity by first break picking, first-kick, peak and trough of theseismic have been picked and the results (Depth models and Velocity models) from thedifferent first-break picks have been compared. Figure 4-12 First break picking on first-kick in Promax 4.3.1.5 Extraction to MatlabThe data of first break picks were extracted and loaded to Matlab for refraction analyses(basic analysis: gradient -intercept method).Junghee Kim 48
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4.3.1.6 Gradient intercept methodThe gradient intercept method discussed in chapter was first used to interpret the pickedtravel times. Because the travel time picks do not fall on a straight line, a line of best fit so-called polyfit was used to approximate a straight line representing the picks in MATLAB(Figure 4-13). The test of error between actual data and data from polyfit are measured inAppendix 7. Figure 4-13 Gradient-intercept method graphThe velocities of the first and second layers (and third layers in some case) are estimatedfrom the slopes of each segment of the plot. The thickness of each layer is also estimatedusing the intercept formulae derived in chapter 3. These velocity and thickness values areplaced at the source position and interpolated with the other values at every source position.The results will be in Chapter 5.Junghee Kim 49
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4.3.2 Advanced refraction analysis (North Line) Figure 4-14 work-flow of advanced refraction analysis in North Line 4.4.2.1 SeisimagerSEG-2 format file is used for this process. Seisimager has two main modules. PickwinTMand PlotrefaTM. PickwinTM helps to conduct first break picking and PlotrefaTM helps toanalyse the data. Seisimager is a tool for refraction analysis. (Seisimager Manual, 2005). Inthis project, the Seisimager has been used. 4.4.2.2 Initial data analysis and quality controlThe data loaded in Seisimager are checked and bad data are removed. The removed datawere equal to the data removed in Promax. Some data in Zen Garden especially S-wavedata had a lot of noise. Some trace did not have any information. Some traces were killed insome cases and some traces were not applied with first break picks by skipping picking inthe trace. Bandpass was considered. However, by concluding the data given are ok withfirst break picking because it can still showing the first break picks even though it is a lotnoisy deep down. 4.4.2.3 Data ProcessingThe data are uploaded to computer and Seisimager processes the seismic data. Usingfunction of PickwinTM, the first arrival times are picked. (Seisimager 2005)Complete analysis process is as following steps. (Anne Obermann 2000) 4.4.2.4 Elevation importing.The elevation data were imported to the Seisimager before processing for the North linewhile for the Zen Garden, the area is regarded as flat area. The elevation was set as 2141 min Zen Garden. .Junghee Kim 50
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4.4.2.5 Amplitude RecoveryThe refraction data may have suffered from amplitude decay due to spherical divergenceand other factors. It is also possible that there have one or two dispersion phenomena in thedata. It is therefore, necessary that before making any pick on the data, some form ofconditioning (which includes amplitude recovery) should be made on the refraction data. Figure 4-15 Original data before applying any form of gain.Figure 4-15 shows the original data as acquired, without any kind of processing applied to it.Obviously, picking on a dataset as this is not practical. The dataset is therefore corrected foramplitude decay, stretched so as to display a few initial times, as we have no need for latearrivals, and finally the amplitudes are clipped to avoid errors in the auto-picker. Figure 4-16shows the corrected form of the same data as figure 4-15. First arrivals picking can now bedone on some data as Figure 4-16. Figure 4-16 Data in figure 4-15 after amplitude correction, stretching.Junghee Kim 51
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4.4.2.6 Travel Time Pick and QCHaving corrected for amplitude, first arrivals are then picked and interpreted. 4.4.2.7 Reciprocal Time CheckA basic principle of refraction seismic method is that time reciprocity is valid, i.e.interchanging the source and the receiver positions does not change the arrival time of therefraction events (Phillip Kearey et al. 2002).The error in the reciprocal time is therefore used a QC test for the quality of picks made.Errors greater than 5% of the traveltime suggests that the pick was bad and as such shouldbe repeated. Figure 4-17 shows a sample of a reciprocal time test made in this project.Clearly the error is minimal and hence suggests that this pick is very good. The test isperformed for the entire line using sets of shots having significant refractoroverlap.(Appendix 8.) Figure 4-17 Reciprocal test for two shots with significant refractor overlap.Junghee Kim 52
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4.4.2.8 First break picks of P-wave in North Line Figure 4-1 Example of P-wave first break picking on first-kickThe whole 7 move-ups have been first break picked and each move-up has been first breakpicked individually. The first break picks of whole 7 move-ups are to show the whole seismicrefraction map and the individual first break picks are for showing individual seismicrefraction image of interesting area. At this time, the first break picks were picked at first kickpoints (Note that the hammer seismic source is impulsive energy which is minimum phase.So, first break picks would be the first energy that is detected.). The first break picks havebeen picked every 3 shot. 4.4.2.9 Advanced Seismic Refraction Analysis using SeisimagerThe travel times picked are interpreted using gradient, reciprocal method (a betterinterpretation method with no assumption of plane interface), Inversions techniques (Timeterm and tomographic). 4.4.2.10 Layer assignmentThe seismic refraction methods such as reciprocal method, time-term inversion are usingthe concept of delay time as discussed in chapter two. The processing software used(Seisimager PlotRefra) relies on the user to assign layers on the travel time picks. Figure 4-19 shows the layer assignment done for one example. It is worth noting that great care hadbeen taken in picking the travel times as the affect the results of any interpretation algorithmstrongly.Junghee Kim 53
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Figure 4-19 Example of layer assignment 4.4.2.11 Reciprocal methodAccording to Jocelyn Dufour and Darren Foltinek (2000), the reciprocal method (in otherwords, delay time method) is developed to solve the time delays of reflection seismic data.Based on the determination of the crossover point and reciprocity, the method is performed.In this project, area of West to East distance 85 m to 144 m in North Line has been chosenfor this analysis since this method can analyse only reciprocal time window area whichshould be chosen. The result is compared with result from the other methods in the NorthLine.Junghee Kim 54
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Figure 4-2 Example of reverse line forming with delay time line for reciprocal methodThe pink line in Figure 4-20 is showing the reduced travel time line generated in Seisimager.It calculates delay time. And optionally, the reverse delay time line is created and does sameprocess and averages the delay time values. With calculated V1 and V2 (when assigned), Itcalculates depth in the each points (P1, P2, … Pn) within reciprocal window according toEquation (2.44) and interpolates those.The result will be shown in Chapter 5. 4.4.2.12 Time term inversionTime-term inversion assumes that the subsurface is vertically stratified and does notconsider the lateral changes during inversion. The depth to the top of the underlying layers iscalculated based on points of first break picking. On the basis of the points assigned fordifferent layers, a layered model is generated. The depth is calculated and interpolated andthe layered model from the time term inversion is generated (Takaya Iwasaki 2002;Seisimager/2D Manual 2005)..In this project, with the values V1 and V2 calculated in Seisimager, depths of every point (P1,P2,.., Pn) in Figure 4-20 are calculated by principle of Equation (2.54) and interpolated.Same process is performed between 2nd layer and 3rd layer if there is 3rd layer.Figure 4-21 shows one example of result of time-term inversion.Junghee Kim 55
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Figure 4-3 Example of Layered model from time-term inversion (from one move-up data of North Line) 4.4.2.13 Tomographic inversionThe tomographic inversion as discussed in chapter three, tries to match the acquired data byiteratively adjusting a model until the misfit between the data created from this model and thereal data is below some acceptable level. The tomographic inversion performed in thisproject uses an initial model generated from time term inversion (Jacob R. Sheehan et al.2000 ; Seisimager/2D Manual 2005)..Tomographic inversion method is fairly sensitive to the initial model. It was thereforenecessary that out results of time term inversion was good enough to start the tomographicinversion. (Jacob R. Sheehan et al. 2000 ; Seisimager/2D Manual 2005).Junghee Kim 56
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Figure 4-4 Process of Tomographic inversion (from one move-up data of North Line)Actual values of matrix To are calculated with layers designed for tomographic inversion.The values of layer lengths get divided and become corresponding to the number of layersdesigned manually to make initial model. To make it clear, let’s assume the number of layers in time-term inversion was 3 and 6layers are designed for tomographic inversion.Junghee Kim 57
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Figure 4-23 Design of the number of layers for initial model As seen Figure 4-23, number of elements in matrix of To became same number as T1 (From3 layers to 6 layers) and it is applied to find ∆S. The number of elements in matrix of ∆S, S1,S2, ….,Sn becomes same number as the number of layers manually designed fortomographic inversion.In this project, to find sensitivity of initial model by parameter (the number of layers, minimumvelocity and maximum velocity) set up was tested before tomographic inversion.And at the point when ∆Y is almost “0” when RMS values do not decrease much anymore,the number of iterations was checked. (note that RMS values are inversely proportional tothe number of iterations ) The chosen value of number of iterations is n for the tomographicinversion.Setting range of Minimum and maximum velocities were tested.After tomographic inversion, ray tracing was performed to show the penetration of the raysused in estimating the synthetic travel time data employed in the tomographic inversionalgorithm.Junghee Kim 58
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Figure 4-24 Ray tracing path in tomographic inversionThrough ray tracing path, the reliability of the data with depth was checked. (note that it isnot possible to sample beyond depth not reachable with ray tracing path with the hammerseismic data. (Jacob R. Sheehan et al. 2000 ; Seisimager/2D Manual 2005)Junghee Kim 59
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4.3.3 Seismic Reflection Data Processing and Analysis in North Line Figure 4-25 work-flow of seismic reflection data processing and analysis in North LineTo generate stack that can be compared with image from refraction processing, basicseismic reflection data processing has been performed in Promax.SEG-Y file is used for this process. With the hammer seismic data in Promax, the seismicreflection data processing is performed. Even though the seismic reaches very shallow, itwould be enough to prove the effect of static correction derived from refraction data in thestack. 4.3.3.1 Refraction MutingThe direct arrival waves and refracted waves dominate data. The amplitudes related to thoseevents are high because they travel closely and are not attenuated. (Jakubowicz 2012)In seismic reflection data processing, refraction and direct arrival are considered as acoherent noise and removed. The refraction muting is applied to these data.Junghee Kim 60
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Figure 4-26 Refraction muting in Promax. (left: before refraction muting, middle: applying refraction muting, right : after refraction muting )Junghee Kim 61
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4.3.3.2 Bandpass FilteringBandpass Filter is applied. Here bandpass filter(s) is a frequency filter(s) to each input traceoperated by the filter algorithm in the frequency domain (Steve H. Danbom, Ph.D., P.G. RiceUniversity ESCI 444). To find out the range of frequency of bandpass, the bandpassparameter tests have been conducted.(note that the attempt to find out the range offrequency of bandpass using the function of FK Spectrum Analysis did not work because inthe analysis window, the signal was highly aliased. This is assumed because the samplingrate is too big. The reason of this assumption is because if KMax of data acquired withhammer are not satisfied with Equation (3.1), the data are aliased. (3.1) Here KMax = Maximum frequency (hz) ∆x = sampling rate (s) (Jakubowicz 2012)The sampling rate was checked in Promax. It was 2.5 ms. The Nyquist Frequency is 1/ 2.5×1000 = 400 hz. The data acquired with hammer must have higher maximum frequency thanthis. Figure 4-27 Aliased reflectors of data in FK spectrum analysisJunghee Kim 62
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Figure 4-5 Schematic drawing on cut range of Bandpass (frequency: 50 – 100 - 200 - 400 Hz)The parameter test was performed. The ranges of frequencies are illustrated in Appendix 9.The bandpass range of 50-100-200-400 was giving the best result keeping reflector the mostand removing the ground roll the most. So this value was chosen.By applying bandpass with frequency range 50-100-200-400, the ground roll wassuccessfully removed and reflector existing in the data seems to reveal.Junghee Kim 63
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Figure 4-6 Bandpass filter application ( left: gather before applying bandpass, right: gather after applying bandpass 4.3.3.3 Static CorrectionIn this project, the final datum was set as 2259 m and replacement velocity was set at 1700m/s in this project. The final datum 2259 m was chosen with the height around 10 m higherthan the highest elevation. The replacement velocity 1700 m/s was chosen with the averagevelocity value of weathering layer.Junghee Kim 64
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4.3.3.3.1 Elevation Statics Analysis in North line. Figure 4-70 schematic geometry for elevation statics with data from first break picks on first-kick of Promax Elevation static correction is calculated as: (3.2) (Khaled Al Dulaijan 2008)In this project, the base of weathering was calculated in Promax with the first break picks onfirst-kick. And the elevation statics have been calculated based on the value, final datumvalue and replacement velocity.Junghee Kim 65
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4.3.3.3.2 Datum static correction from tomographic inversion of Seisimager in Promax: Figure 4-8 schematic geometry for datum statics using data from tomographic inversion of SeisimagertLVL is calculated as: (3.3) (Khaled Al Dulaijan 2008)The elevation static correction is calculated as: (3.4) (Khaled Al Dulaijan 2008)Junghee Kim 66
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